Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A mobile communications device with a wireless module and a controller
module is provided. The wireless module selectively performs wireless
transceiving to and from a first service network or a second service
network. The controller module receives, via the wireless module, a
handover request for a handover from the first service network to the
second service network, and determines whether a Non-Access Stratum (NAS)
message flow with a counted sequence number is active on a
Circuit-Switched (CS) domain of the first service network in response to
the handover request. Also, the controller module keeps the counted
sequence number unchanged in response to the NAS message flow being
active on the CS domain of the first service network. Specifically, the
handover request is transmitted by the first service network in response
to a Single Radio Voice Call Continuity (SRVCC) procedure between the
first service network and the second service network.

Claims:

1. A mobile communications device, comprising: a wireless module
selectively performing wireless transceiving to and from a first service
network or a second service network; and a controller module receiving,
via the wireless module, a handover request for a handover from the first
service network to the second service network, determining whether a
Non-Access Stratum (NAS) message flow with a counted sequence number is
active on a Circuit-Switched (CS) domain of the first service network in
response to the handover request, and keeping the counted sequence number
unchanged in response to the NAS message flow being active on the CS
domain of the first service network, wherein the handover request is
transmitted by the first service network in response to a Single Radio
Voice Call Continuity (SRVCC) procedure between the first service network
and the second service network.

2. The mobile communications device of claim 1, wherein the controller
module further performs the handover from the first service network to
the second service network according to the handover request, and the
keeping the counted sequence number unchanged is performed in response to
the handover being successfully performed.

3. The mobile communications device of claim 1, wherein the controller
module further initializes the counted sequence number to 0 in response
to no NAS message flow being active on the CS domain of the first service
network.

4. The mobile communications device of claim 3, wherein the
initialization of the counted sequence number is performed in response to
the handover being successfully performed.

6. The mobile communications device of claim 1, wherein the counted
sequence number is incremented by one for each transmitted message in the
NAS message flow.

7. The mobile communications device of claim 1, wherein the first service
network operates in compliance with a High Speed Packet Access (HSPA)
mobile telephony protocol, and the second service network operates in
compliance with one of the following mobile telephony protocols: Wideband
Code Division Multiple Access (WCDMA); General Packet Radio Service
(GPRS); and Global System for Mobile Communications (GSM).

8. A method for a mobile communications device to control sequenced
message transfer during a Single Radio Voice Call Continuity (SRVCC)
procedure, comprising: receiving a handover request for a handover from a
first service network to a second service network; determining whether a
Non-Access Stratum (NAS) message flow with a counted sequence number is
active on a Circuit-Switched (CS) domain of the first service network;
and keeping the counted sequence number unchanged in response to the NAS
message flow being active on the CS domain of the first service network.

9. The method of claim 8, further comprising performing a handover from
the first service network to the second service network according to the
handover request, wherein the keeping the counted sequence number
unchanged is performed in response to the handover being successfully
performed.

10. The method of claim 8, further comprising initializing the counted
sequence number to 0 in response to no NAS message flow being active on
the CS domain of the first service network.

11. The method of claim 10, wherein the initialization of the counted
sequence number is performed in response to the handover being
successfully performed.

12. The method of claim 8, wherein the NAS message flow is a Mobility
Management (MM), Call Control (CC), or Supplementary Service (SS) message
flow.

13. The method of claim 8, wherein the counted sequence number is
incremented by one for each transmitted message in the NAS message flow.

14. The method of claim 8, wherein the first service network operates in
compliance with a High Speed Packet Access (HSPA) mobile telephony
protocol, and the second service network operates in compliance with one
of the following mobile telephony protocols: Wideband Code Division
Multiple Access (WCDMA); General Packet Radio Service (GPRS); and Global
System for Mobile Communications (GSM).

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims priority of U.S. Provisional Application
No. 61/315,249, filed on Mar. 18, 2010, and the entirety of which is
incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention generally relates to the technique of sequenced
message transfer operation, and more particularly, to apparatuses and
methods for controlling sequenced message transfer during a Single Radio
Voice Call Continuity (SRVCC) procedure.

[0004] 2. Description of the Related Art

[0005] With growing demand for mobile communications, the Global System
for Mobile communications (GSM) supporting only Circuit-Switched (CS)
domain services no longer meets user requirements. The mobile
communications working groups and standard groups accordingly have
developed the so-called third generation mobile communications
technologies, such as the Wideband Code Division Multiple Access (WCDMA)
technology. Taking the Universal Mobile Telecommunications System (UMTS)
using the WCDMA technology for example, the 3rd Generation Partnership
Project (3GPP) has further proposed the Long Term Evolution (LTE) system,
also called the fourth generation mobile communications system, to meet
future demand for large wireless data transmission bandwidths. The LTE
system aims to provide an all-IP architecture, in which only
Packet-Switched (PS) domain instead of CS domain is used to carry all
mobile communications services. Also, voice services are provided by
Voice over Internet Protocol (VoIP) in the fourth generation mobile
communications system, instead of the CS domain of the third generation
mobile communications system.

[0006] However, during transition from the third generation mobile
communications system to the fourth generation mobile communications
system, some operators have still chosen to provide voice services via
the CS domain of the third generation mobile communications system, as
they believe the coverage rate and capacity of the CS domain of the third
generation mobile communications system are sufficient and the cost to
provide ubiquitous LTE coverage is too high. Thus, a problem arises,
wherein the architecture of the third generation mobile communications
system comprises both the CS domain and the PS domain while the fourth
generation mobile communications system comprises only the PS domain.
Accordingly, a new function, called Single Radio Voice Call Continuity
(SRVCC), has been specified for the interconnection between the third
generation mobile communications system and the fourth generation mobile
communications system, so that a voice call may smoothly be transferred
from the PS domain of the fourth generation mobile communications system
to the CS domain of the third generation mobile communications system.

[0007] Before SRVCC is initiated, there may be Non-Access Stratum (NAS)
message flows ongoing in a mobile communications device, which are
protected using the sequenced message transfer operation. That is, a
counted sequence number is maintained for each ongoing NAS message flow,
which incrementally counts the sequence number of the next message to be
transmitted in the associated NAS message flow. By comparing the sequence
number of a currently received message with the sequence number of the
last received message, the mobile communications system may determine
whether the currently received message is a new transmission or a
retransmission. Note that, the sequenced message transfer operation may
be reset when SRVCC is initiated and completed successfully, which causes
the counted sequence number to be reinitialized to 0. In this situation,
the mobile communications system may discard the message received after
SRVCC when detecting that the sequence number of the message is out of
sequence. As a result, abnormal unsmoothness or even breaking off of the
services carried by the ongoing NAS message flows may occur.

BRIEF SUMMARY OF THE INVENTION

[0008] In one aspect of the invention, a mobile communications device
comprising a wireless module and a controller module is provided. The
wireless module selectively performs wireless transceiving to and from a
first service network or a second service network. The controller module
receives, via the wireless module, a handover request for a handover from
the first service network to the second service network, and determines
whether a Non-Access Stratum (NAS) message flow with a counted sequence
number is active on a Circuit-Switched (CS) domain of the first service
network in response to the handover request. Also, the controller module
keeps the counted sequence number unchanged in response to the NAS
message flow being active on the CS domain of the first service network,
wherein the handover request is transmitted by the first service network
in response to a Single Radio Voice Call Continuity (SRVCC) procedure
between the first service network and the second service network.

[0009] In another aspect of the invention, a method for a mobile
communications device to control sequenced message transfer during an
SRVCC procedure is provided. The method comprises the steps of receiving
a handover request for a handover from a first service network to a
second service network, determining whether an NAS message flow with a
counted sequence number is active on a CS domain of the first service
network, and keeping the counted sequence number unchanged in response to
the NAS message flow being active on the CS domain of the first service
network.

[0010] Other aspects and features of the present invention will become
apparent to those with ordinarily skill in the art upon review of the
following descriptions of specific embodiments of apparatuses, systems,
and methods for controlling sequenced message transfer during an SRVCC
procedure.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The invention can be more fully understood by reading the
subsequent detailed description and examples with references made to the
accompanying drawings, wherein:

[0012] FIG. 1 is a block diagram illustrating a mobile communications
environment according to an embodiment of the invention;

[0013] FIG. 2 is a message sequence chart illustrating a sequenced message
transfer involving an active NAS message flow on the CS domain during an
SRVCC procedure according to an embodiment of the invention;

[0014] FIG. 3 is a message sequence chart illustrating a sequenced message
transfer involving an inactive NAS message flow on the CS domain during
an SRVCC procedure according to an embodiment of the invention;

[0015]FIG. 4 is a flow chart illustrating a method for controlling
sequenced message transfer during an SRVCC procedure according to an
embodiment of the invention; and

[0016]FIG. 5 is a flow chart illustrating a method for controlling
sequenced message transfer during an SRVCC procedure according to another
embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The following description is of the best-contemplated mode of
carrying out the invention. This description is made for the purpose of
illustrating the general principles of the invention and should not be
taken in a limiting sense. The 3GPP specifications are used to teach the
spirit of the invention, and the invention is not limited thereto.

[0018] FIG. 1 is a block diagram illustrating a mobile communications
environment according to an embodiment of the invention. In the mobile
communications environment 100, the mobile communications device 110 may
be selectively connected to one of the service networks 120 and 130,
wherein the service networks 120 and 130 are both connected to an IMS 140
for anchoring IMS multimedia telephone sessions. In this embodiment, the
service network 120 is a UMTS system using the HSPA technology (herein
referred to as a HSPA system) which comprises a Universal Terrestrial
Radio Access Network (UTRAN) 121 for providing the functionality of
wireless transceiving for the service network 120, a Mobile Switching
Center (MSC) 122 for routing voice calls, Short Message Service (SMS),
and circuit switched data, etc., and a Serving GPRS (General Packet Radio
Service) Support Node (SGSN) 123 for handling the packet switched data
within the service network 120, e.g. the mobility management and
authentication of the users. The service network 130 may be a GSM system
which comprises a Base Station Subsystem (BSS) 131 for providing the
functionality of wireless transceiving for the service network 130, and
an MSC 132 for routing voice calls, SMS, and circuit switched data, etc.
Specifically, the mobile communications device 110 is in an area under
the overlapped radio coverage of the service networks 120 and 130. The
mobile communication device 110 comprises a wireless module 111 for
performing the functionality of wireless transceiving to and from one of
the service networks 120 and 130. To further clarify, the wireless module
111 may comprise a baseband unit (not shown) and a radio frequency (RF)
unit (not shown). The baseband unit may contain multiple hardware devices
to perform baseband signal processing, including analog to digital
conversion (ADC)/digital to analog conversion (DAC), gain adjusting,
modulation/demodulation, encoding/decoding, and so on. The RF unit may
receive RF wireless signals, convert the received RF wireless signals to
baseband signals, which are processed by the baseband unit, or receive
baseband signals from the baseband unit and convert the received baseband
signals to RF wireless signals, which are later transmitted. The RF unit
may also contain multiple hardware devices to perform radio frequency
conversion. For example, the RF unit may comprise a mixer to multiply the
baseband signals with a carrier oscillated in the radio frequency of the
wireless communications system, wherein the radio frequency may be may be
900 MHz, 1800 MHz or 1900 MHz utilized in GSM systems, or may be 900 MHz,
1900 MHz, or 2100 MHz utilized in HSPA systems, or others depending on
the radio access technology (RAT) in use. Also, the mobile communication
device 110 comprises a controller module 112 for controlling the
operation of the wireless module 111 and other functional components,
such as a display unit and/or keypad serving as the MMI (man-machine
interface), a storage unit storing the program codes of applications or
communication protocols, or others. In this embodiment, the mobile
communications device 110 may be a User Equipment (UE) in compliance with
the 3GPP TS 23.216 specification, v.9.1.0 (referred to herein as the
23.216 specification), the 3GPP TS 24.007 specification, v.9.0.0
(referred to herein as the 24.007 specification), and/or other related
specifications of the HSPA and GSM technologies.

[0019] To be more specific, the controller module 112 controls the
wireless module 111 for performing sequenced message transfer during an
SRVCC procedure. FIG. 2 is a message sequence chart illustrating a
sequenced message transfer involving an active NAS message flow on the CS
domain during an SRVCC procedure according to an embodiment of the
invention. In this embodiment, the mobile communications device 110 is
initially connected to the service network 120 for obtaining Voice over
IP (VoIP) call service. In addition to the VoIP call service, there is
also an NAS message flow on the CS domain between the mobile
communications device 110 and the service network 120. Specifically, the
NAS message flow on the CS domain may be protected by the sequenced
message transfer operation, i.e. a counted sequence number is maintained
for the NAS message flow on the CS domain, which may be referred to as a
send-state variable, V(D), and incrementally counts the sequence number
of the next message to be transmitted in the associated NAS message flow.
In one embodiment, the NAS message flow on the CS domain may be
established for a Mobility Management (MM) procedure. The MM procedure
may include an authentication procedure, identification procedure,
Temporary Mobile Subscriber Identity (TMSI) reallocation procedure, MM
information procedure, and/or abort procedure. In other embodiments, the
NAS message flow on the CS domain may be established for a Call Control
(CC) procedure, or Supplementary Service (SS) procedure. The CC procedure
may be initiated for decoding address information and/or routing
telephone calls, and the SS procedure may be initiated for providing
feature services, such as call waiting, call forwarding on busy, and no
disturbing, etc. Alternatively, the NAS message flow on the CS domain may
be established for any combination of the MM, CC, and SS procedures.
Detailed descriptions of the MM, CC, and SS procedures are omitted here
for brevity as they are beyond the scope of the invention. Note that, in
this embodiment, as the mobile communications device 110 moves closer to
the service network 130, the detected signal quality of the service
network 120 falls below a predetermined threshold while the detected
signal quality of the service network 130 is greater than the
predetermined threshold (step S205). The mobile communications device 110
then reports on the detected signal qualities of the service networks 120
and 130 to the service network 120 according to the measurement
configuration given by the service network 120 beforehand (step S210).
When receiving the measurement report, the service network 120 decides to
hand over the mobile communications device 110 to the service network 130
(step S215), and then the SGSN 123 initiates an SRVCC procedure with the
MSC 132 (step S220). In another embodiment, after initiating the SRVCC
procedure, the SGSN 123 may perform the handover of the non-voice PS
bearer if any non-voice PS bearer exists between the service network 120
and the mobile communications device 110. In response to the initiation
of the SRVCC procedure, the MSC 132 establishes a bearer path for the
mobile communications device 110 in the service network 130 (step S225),
and notifies the IMS 140 that the voice call for the mobile
communications device 110 needs to be moved from the PS domain to the CS
domain, to enable the IMS 140 to perform a packet-to-circuit interworking
function for the voice call upon notification by the MSC 132 (step S230).
When the transfer of the voice call from the PS domain to the CS domain
is completed, the MSC 132 replies to the SGSN 123 with a
packet-to-circuit handover response (step S235). The SGSN 123 transmits a
handover request to the mobile communications device 110 via the UTRAN
121 in response to the packet-to-circuit handover response (step S240).
For the handover request indicating handover from the service network 120
to the service network 130, the controller module 112 detects that an NAS
message flow with a counted sequence number exists on the CS domain of
the first service network 120 (step S245), and further determines whether
the NAS message flow is active (step S250). In this embodiment, due to
that the NAS message flow being established for an ongoing SS procedure,
the controller module 112 then keeps the counted sequence number
unchanged (step S255), so that the ongoing SS procedure may run smoothly
after the SRVCC procedure. Lastly, the controller module 112 switches the
internal voice processing from VoIP call to CS voice call when the mobile
communications device 110 arrives on-channel in the service network 130
(step S260), wherein the voice call may be continued.

[0020] FIG. 3 is a message sequence chart illustrating a sequenced message
transfer involving an inactive NAS message flow on the CS domain during
an SRVCC procedure according to an embodiment of the invention. Similar
to the embodiment of FIG. 2, the mobile communications device 110 is
initially connected to the service network 120 for obtaining an IMS
emergency call service. In addition to the IMS emergency call service,
there is also an NAS message flow on the CS domain between the mobile
communications device 110 and the service network 120. Specifically, the
NAS message flow on the CS domain may be protected by the sequenced
message transfer operation, i.e. a counted sequence number is maintained
for the NAS message flow on the CS domain, which may be referred to as a
send-state variable, V(D), and incrementally counts the sequence number
of the next message to be transmitted in the associated NAS message flow.
In one embodiment, the NAS message flow on the CS domain may be
established for an MM or CC procedure. The MM procedure may include an
authentication procedure, identification procedure, TMSI reallocation
procedure, MM information procedure, and/or abort procedure. The CC
procedure may be initiated for decoding address information and/or
routing telephone calls. In another embodiment, the NAS message flow on
the CS domain may be established for any combination of the MM and CC
procedures. Detailed descriptions of the MM and CC procedures are omitted
here for brevity as they are beyond the scope of the invention. In this
embodiment, as the mobile communications device 110 moves closer to the
service network 130, the detected signal quality of the service network
120 falls below a predetermined threshold while the detected signal
quality of the service network 130 is greater than the predetermined
threshold (step S305). The mobile communications device 110 then reports
on the detected signal qualities of the service networks 120 and 130 to
the service network 120 according to the measurement configuration given
by the service network 120 beforehand (step S310). When receiving the
measurement report, the service network 120 decides to hand over the
mobile communications device 110 to the service network 130 (step S315),
and then the SGSN 123 initiates an SRVCC procedure with the MSC 132 (step
S320). In another embodiment, after initiating the SRVCC procedure, the
SGSN 123 may perform the handover of the non-voice PS bearer if any
non-voice PS bearer exists between the service network 120 and the mobile
communications device 110. In response to the initiation of the SRVCC
procedure, the MSC 132 establishes a bearer path for the mobile
communications device 110 in the service network 130 (step S325), and
notifies the IMS 140 that the voice call for the mobile communications
device 110 needs to be moved from the PS domain to the CS domain, to
enable the IMS 140 to perform a packet-to-circuit interworking function
for the voice call upon notification by the MSC 132 (step S330). When the
transfer of the voice call from the PS domain to the CS domain is
completed, the MSC 132 replies to the SGSN 123 with a packet-to-circuit
handover response (step S335). The SGSN 123 transmits a handover request
to the mobile communications device 110 via the UTRAN 121 in response to
the packet-to-circuit handover response (step S340). For the handover
request indicating handover from the service network 120 to the service
network 130, the controller module 112 detects that an NAS message flow
with a counted sequence number exists on the CS domain of the first
service network 120 (step S345), and further determines whether the NAS
message flow is active (step S350). In this embodiment, due to the fact
that the NAS message flow has been previously established for a CC
procedure but is inactive for now, the controller module 112 then
initializes the counted sequence number to 0 (step S355). Lastly, the
controller module 112 switches the internal voice processing from
VoIP/IMS call to CS voice call when the mobile communications device 110
arrives on-channel in the service network 130 (step S360), so that the
voice call may continue.

[0021] Note that, although only one NAS message flow is described in the
embodiments of FIGS. 2 and 3, there may be more than one NAS message flow
which is active or inactive during the SRVCC procedure, and the invention
is not limited thereto. For example, in addition to the NAS message flow
on the CS domain described above, there may be one or more NAS message
flows on the PS domain, which may be established for a Group Call Control
(GCC), Broadcast Call Control (BCC), and/or Control-plane Location
Services (LCS) procedures. For these cases, the controller module 112 may
also initialize each of the counted sequence numbers associated with the
NAS message flows on the PS domain to 0 during the SRVCC procedure. In
addition, although the embodiments of FIGS. 2 and 3 describe the
sequenced message transfers during an SRVCC procedure from an HSPA system
to a GSM system, the invention may also be applied to the sequenced
message transfer during an SRVCC procedure from an HSPA system to any
legacy system, such as a WCDMA system or a GPRS system.

[0022]FIG. 4 is a flow chart illustrating a method for controlling
sequenced message transfer during an SRVCC procedure according to an
embodiment of the invention. In this embodiment, the method for
controlling sequenced message transfer during an SRVCC procedure may be
applied in a mobile communications device for transferring a voice call
from a first service network to a second service network. Initially, the
mobile communications device is connected to the first service network
for obtaining VoIP call service. In addition to the VoIP call service,
there is also an NAS message flow on the CS domain between the mobile
communications device and the first service network. Specifically, the
NAS message flow on the CS domain is protected by the sequenced message
transfer operation, i.e. a counted sequence number is maintained for the
NAS message flow on the CS domain, which may be referred to as a
send-state variable, V(D), and incrementally counts the sequence number
of the next message to be transmitted in the associated NAS message flow.
The NAS message flow on the CS domain may be established for an MM, CC,
or SS procedure. The MM procedure may include an authentication
procedure, identification procedure, TMSI reallocation procedure, MM
information procedure, and/or abort procedure. The CC procedure may be
initiated for decoding address information and/or routing telephone
calls, and the SS procedure may be initiated for providing feature
services, such as call waiting, call forwarding on busy, and no
disturbing, etc. Alternatively, the NAS message flow on the CS domain may
be established for any combination of the MM, CC, and SS procedures.
Detailed descriptions of the MM, CC, and SS procedures are omitted here
for brevity as they are beyond the scope of the invention. To begin, the
mobile communications device receives a handover request for handing over
from the first service network to the second service network (step S410).
In response to the handover request, the mobile communications device
determines whether an NAS message flow with a counted sequence number is
active on the CS domain (step S420). If so, the mobile communications
device keeps the counted sequence number unchanged (step S430). Thus, the
ongoing MM, CC, or SS procedure may run smoothly after the SRVCC
procedure.

[0023]FIG. 5 is a flow chart illustrating a method for controlling
sequenced message transfer during an SRVCC procedure according to another
embodiment of the invention. In this embodiment, the method for
controlling sequenced message transfer during an SRVCC procedure may be
applied in a mobile communications device for transferring a voice call
from an HSPA system to a GSM system. Initially, the mobile communications
device is connected to the HSPA system for obtaining an IMS emergency
call service. To begin, the mobile communications device first detects
that the signal quality of the HSPA system falls below a predetermined
threshold while the signal quality of the GSM system is greater than the
predetermined threshold (step S510). The mobile communications device
then reports on the signal qualities of the HSPA and GSM systems to the
HSPA system (step S520). When receiving the measurement report, the HSPA
system decides to hand over the mobile communications device to the GSM
system, and initiates an SRVCC procedure with the GSM system to move the
voice call from the PS domain of the HSPA system to the CS domain of the
GSM system. After the transfer of the voice call is completed, the mobile
communications device receives a handover request from the HSPA system
(step S530). In response to the handover request, the mobile
communications device determines whether an NAS message flow with a
counted sequence number exists and is active on the CS domain of the HSPA
system (step S540). Specifically, the NAS message flow on the CS domain
of the HSPA system is protected by the sequenced message transfer
operation, i.e. a counted sequence number is maintained for the NAS
message flow on the CS domain of the HSPA system, which may be referred
to as a send-state variable, V(D), and incrementally counts the sequence
number of the next message to be transmitted in the associated NAS
message flow. The NAS message flow on the CS domain may be established
for an MM or CC procedure. The MM procedure may include an authentication
procedure, identification procedure, TMSI reallocation procedure, MM
information procedure, and/or abort procedure, and the CC procedure may
be initiated for decoding address information and/or routing telephone
calls. Alternatively, the NAS message flow on the CS domain of the HSPA
system may be established for any combination of the MM and CC
procedures. Detailed descriptions of the MM and CC procedures are omitted
here for brevity as they are beyond the scope of the invention. If an NAS
message flow with a counted sequence number exists and is active on the
CS domain of the HSPA system, the mobile communications device keeps the
counted sequence number unchanged (step S550), so that the NAS message
flow may continue smoothly after the SRVCC procedure. Otherwise, if no
NAS message flow with a counted sequence number exists on the CS domain
of the HSPA system or the NAS message previously established on the CS
domain of the HSPA system is inactive for now, the mobile communications
device initializes the counted sequence number to 0 (step S560).
Subsequent to steps S550 and S560, the mobile communications device
switches the internal voice processing from IMS call to CS voice call
when handing over from the HSPA system to the GSM system (i.e. when
arriving on-channel in the GSM system) (step S570), and the SRVCC
procedure ends. Note that, although only one NAS message flow is
described above, there may be more than one NAS message flow which is
active or inactive during the SRVCC procedure, and the invention is not
limited thereto. For example, in addition to the NAS message flow on the
CS domain of the HSPA system described above, there may be one or more
NAS message flows on the PS domain of the HSPA system, which may be
established for a GCC, BCC, and/or LCS procedure. For this case, the
mobile communications device may also initialize each of the counted
sequence numbers associated with the NAS message flows on the PS domain
of the HSPA system to 0 during the SRVCC procedure. In addition, although
the embodiment describes the sequenced message transfer during an SRVCC
procedure from an HSPA system to a GSM system, the invention may also be
applied to the sequenced message transfer during an SRVCC procedure from
an HSPA system to any legacy system, such as a WCDMA system or a GPRS
system.

[0024] While the invention has been described by way of example and in
terms of preferred embodiment, it is to be understood that the invention
is not limited thereto. Those who are skilled in this technology can
still make various alterations and modifications without departing from
the scope and spirit of this invention. Therefore, the scope of the
present invention shall be defined and protected by the following claims
and their equivalents.